1. Field of the Invention
The present invention relates to a novel carbon black, an electrocatalyst carrier formed from this carbon black, an electrocatalyst using such a carrier, and an electrochemical device such as a solid polymer electrolyte fuel cell using such an electrocatalyst.
2. Description of the Background Art
Solid polymer electrolyte fuel cells enable high current densities to be drawn at low temperatures, and as a result, such fuel cells are currently being developed as portable power supplies, drive sources for electric vehicles, and as cogeneration power supplies.
A solid polymer electrolyte fuel cell comprises an ion exchange membrane formed from the solid polymer electrolyte positioned between a fuel electrode (anode) and an air electrode (cathode). Both the fuel electrode and the air electrode are formed from a mixture of a catalyst comprising a supported noble metal and a polymer electrolyte.
According to such a structure, in the case of hydrogen as the fuel, hydrogen gas passes through pores in the fuel electrode and reaches the catalyst, and then emits electrons under the effect of the catalyst, forming hydrogen ions. These hydrogen ions are transported through the electrolyte in the electrode and the solid polymer electrolyte membrane between the two electrodes to the air electrode. The emitted electrons pass through the catalyst carrier within the electrode and flow into an external circuit, and then travel through the external circuit to the air electrode. In contrast, in the case of oxygen as the oxidant, oxygen passes through pores in the air electrode and reaches the catalyst, and then reacts with the hydrogen ions and the electrodes transported from the fuel electrode to generate water.
In a solid polymer electrolyte fuel cell, in order to accelerate the electrode reactions and improve the characteristics of the cell, the catalytic activity of the catalyst within the electrode must first be as high as possible. As a result, catalysts in which a highly active noble metal, and in particular platinum or a platinum alloy, is supported on a carbon black carrier are widely used.
In addition, in order to ensure the most efficient use of these very expensive noble metal catalysts, the contact surface area between the catalyst and the polymer electrolyte within the electrode must be increased. Furthermore, in order to reduce concentration overvoltage arising from the delay in gas supply to noble metal catalyst positioned in regions distant from the gas flow, the diffusion of gases (hydrogen, oxygen) supplied to the electrode reactions must be maximized.
Accordingly, much research is being conducted on both methods of supporting noble metals, and catalyst carriers. For example, as a method of improving the contact between the electrode catalyst and the polymer electrolyte, Japanese Laid-open publication (kokai) No. Hei 9-167622 (JP9-167622A) discloses a method for controlling the adsorption of noble metal particles within pores to which the polymer electrolyte cannot be distributed, by supporting the noble metal using a carbon black carrier in which the volume of pores with a diameter of no more than 8 nm is not more than 500 cm3/g. Furthermore, using a similar approach, Japanese Laid-open publication (kokai) No. 2000-100448 (JP2000-100448A) discloses that using carbon black in which those pores with a diameter of less than 6 nm account for no more than 20% of total pores as a carrier is effective in improving the catalyst utilization.
Japanese Unexamined Laid-open publication (kokai) No. Hei 6-203840 (JP6-203840A) discloses that increasing the percentage of voids within the catalyst layer from 65 to 90% by volume is effective in improving the diffusion of reactant gas at the electrodes. In addition, other methods for improving the characteristics of a solid polymer electrolyte fuel cell include using an electrode in which the volume of pores with diameters within a range from 0.04 to 1 μm is at least 0.06 cm3/g, as disclosed in Japanese Laid-open publication (kokai) No. Hei 9-92293 (JP9-92293A), and similarly using an electrode in which the volume of pores with diameters of greater than 0.1 μm is at least 0.4 cm3/g, as disclosed in Japanese Laid-open publication (kokai) No. Hei 9-283154 (JP9-283154A).
In addition, Japanese Laid-open publication (kokai) No. Hei 6-203852 (JP6-203852A) discloses a method for ensuring sufficient pores for reactant gas diffusion within the electrode by adding a pore forming material during production of the electrode. Furthermore, J. Appl. Electrochem., Vol. 28 (1998) pp. 277 reports that addition of a pore forming material improves gas diffusion at the air electrode and improves the characteristics of a solid polymer electrolyte fuel cell.